exec: Remove the stack gap implementation

ASLR stack randomization will reappear in a forthcoming commit. Rather
than inserting a random gap into the stack mapping, the entire stack
mapping itself will be randomized in the same way that other mappings
are when ASLR is enabled.

No functional change intended, as the stack gap implementation is
currently disabled by default.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 758d98debec43ff83b8a1ed9a3d3a8441b83b3cc)